Homework Help:
Another force problem

A 57.0 kg girl weighs herself by standing on a scale in an elevator. What is the force exerted by the scale when the elevator is descending at a constant speed of 10 m/s?
What is the force exerted by the scale if the elevator is accelerating downward with an acceleration of 2.0 m/s2?
If the elevator's descending speed is measured at 10 m/s at a given point, but its speed is decreasing by 2.0 m/s2, what is the force exerted by the scale?

2. Relevant equations
F=ma, Fs-W=ma

3. The attempt at a solution
I got the first part, 9.8(57) = 558.6 N.

For the second part I used Fs-W=ma, Fs - 558.6 = (57)2, Fs= 672 N. But that was wrong...?

For the 3rd part, I used F=ma, but F= 57 (-2) = -144 N, and that wasn't right either.

You are making sign mistakes. For the second part write Fs+W=ma. Fs is always in the up direction, so lets call that the + direction. The weight acts down so I should put that in with a minus sign, ditto acceleration is down, so put that in with a minus sign. Fs-558.6N=(57kg)*(-2)m/s^2. Now what do you get? What happens in part three?

The elevators downward speed is DECREASING. This means its acceleration is actually UPWARDS at 2m/sec^2. You do the same thing with the 10m/sec that you did in part one. The current speed doesn't have anything to do with the current acceleration.